1. Field of the Invention
This invention relates to a rotary atomizing head type coating machine particularly suitable for use in paint coating operations involving color changes.
2. Discussion of the Background
Generally, rotary atomizing head type coating machines are largely constituted by: a cover which is formed in a cylindrical shape; an air motor which is provided within the cover; a rotational shaft which is provided axially in and rotated by the air motor; a rotary atomizing head which is mounted axially in the rotational shaft and put in high-speed rotation by the rotational shaft; and a feed tube provided axially in the rotational shaft, and having a fore end extended into the rotary atomizing head for spouting thereinto a paint or a thinner as a solvent.
In turn, the above-mentioned rotary atomizing head is constituted by: a bell cup which is formed in a bell- or cup-like shape; a hub member which is located on the inner peripheral side of the bell cup and defines thereon a paint reservoir for holding a pool of paint supplied through the paint feed tube; a plural number of paint outlet holes provided on the outer peripheral side of the hub member to permit the paint supplied through the feed tube to flow out from the paint reservoir toward marginal releasing edge at the fore end of inner peripheral surface of the bell cup; and a plural number of solvent outlet holes provided in center portion of the hub member to permit the thinner supplied through the feed tube to flow out from the paint reservoir toward the front side of the hub member.
The above-mentioned feed tube is connected to a color changing valve device which supply paint, air and thinner through a paint supply pipe. Further, the coating machine is connected to a high voltage generator to apply negative high voltage in electrostatic coating operations.
With a conventional rotary atomizing head type coating machine which is arranged in this manner, a coating operation is started firstly by supplying compressed air to the air motor to put the rotary atomizing head in high speed rotation together with the rotational shaft. In the next place, a paint is spouted out from the feed tube into the paint reservoir which is provided on the rotary atomizing head. As a result, the paint is urged to flow along inner peripheral surface of the bell cup and, after being spread into a thin film, sprayed as charged paint particles from the marginal releasing edge at the fore end of the bell cup. At this time, the change paint particles, which are released from at marginal edge of the rotary atomizing head, are urged to fly toward a coating object along lines of electric force of an electrostatic field which is formed between the coating object, and to deposit on the coating object.
Further, it becomes necessary to change the paint color, air and thinner are supplied to the rotary atomizing head from the color changing valve device, thereby washing away the previous color which has deposited on liquid-contacting surfaces of the rotary atomizing head. Then, a paint of a different color is supplied to the paint supply passage in preparation for a coating operation with a new color.
However, at the time of supplying paint of a fresh color as described above, it is often the case that the new color deposits on front surface of the hub member as well as on outer peripheral surface of the bell cup. Besides, the paint which has deposited on the rotary atomizing head undergoes solidification while a coating line is stopped for a certain time period due to a trouble on the line or for a lunch-time break. Similarly, paint deposition and accumulation takes place when a paint of same color is used for hours.
In such a case, it becomes necessary to wash solidified paint deposits off the liquid-contacting surfaces of the rotary atomizing head since otherwise defoliated fragments of solidified paint will make coating defects.
In this regard, in order to wash deposited paint off the rotary atomizing head, while the rotational shaft and rotary atomizing head are rotated by the air motor, thinner is spurted out from the feed tube into the paint reservoir on the rotary atomizing head, separately from air and thinner which is supplied from the color changing valve device. By so doing, a part of the thinner is supplied to the marginal releasing edge through the paint outlet holes from the paint reservoir to wash away the deposited paint at the marginal releasing edge. Also, a part of the thinner is supplied onto front surface of the hub member through the solvent outlet holes to wash away deposited paint from front surface of the hub member.
In a coating operation with a conventional paint coating machine of this sort, paint particles which are sprayed from marginal releasing edge of the bell cup are applied with a high voltage by a high voltage generator, and most of the charged paint particles are urged to fly along an electrostatic field toward a coating object which is connected to the earth. However, there is often a case that a part of the released paint particles tend to flow in an inverse direction toward the rear side of the bell cup, and to deposit on fore end portions of outer peripheral surface of the bell cup.
Namely, when the rotary atomizing head is put in high speed rotation, vacuum pressure regions occur on the front side of the bell cup under the influence of the high speed rotation, and a part of paint particles are sucked into the vacuum regions by the so-called pumping phenomenon and caused to flow in a reversed direction toward the rear side of the bell cup.
In addition, for the purpose of shaping the spray of paint particles into a suitable pattern depending upon the conditions of coating operation, shaping air is spurted toward the outer peripheral side of the rotary atomizing head from shaping air outlets which are located at the front end of the cover. Due to the jet streams of shaping air, vacuum pressures are developed partially around the outer peripheral side of the bell cup, and as a result, a part of paint particles are caused to flow inversely toward the rear side of the bell cup.
If paint particles are partly entrained on inverse air flows in this manner, they deposit on outer peripheral surface of the bell cup and remain there in a solidified state. Therefore, under certain conditions of coating operation, solidified paint falls off in small fragments which can detrimentally impair the quality of end products by depositing on coated surfaces.
Further, according to the prior art coating machine, the marginal releasing edge of the bell cup and front surface of the hub member are washed with a thinner which is spurted out toward the inner peripheral surface of the bell cup from the afore-mentioned feed tube. However, difficulties are often experienced in removing deposited paint from the outer peripheral surface of the bell cup simply by supplying a thinner through a feed tube in such a manner.
In this connection, in an attempt to overcome the problem just mentioned, Japanese Utility Model Laid Open No. S57-62659 (hereinafter referred to as "other prior art" for brevity) discloses a coating machine employing a washing nozzle which is arranged to spurt a thinner toward the outer peripheral surface of a bell cup for washing away deposited paint therefrom.
According to the coating machine by the other prior art just mentioned, at the time of washing the outer peripheral side of a bell cup, a thinner is simply spurted toward outer peripheral surface of the bell cup while the rotary atomizing head is being kept in rotation. Therefore, the thinner is splashed back on outer peripheral surface of the bell cup instead of being brought into intimate contact with the outer peripheral surface thereof, there by often failing to washing away deposited paint from fore end portions of outer peripheral surface of the bell cup in an assured manner.
In order to wash away deposited paint from outer peripheral surface of the bell cup in a more reliable manner, free of the above-mentioned problems of the thinner being splashed back without contacting outer peripheral surface of the bell cup to a sufficient degree, it is necessary to determine the position and direction of a washing nozzle precisely and elaborately in relation with a thinner feed rate through engineering processes which require extremely sophisticated machine designing and production technology.
Further, considering complications in construction and increases in the number of machine parts and production cost, it is inefficient and uneconomical to provide a washer nozzle exclusively for the purpose of washing outer peripheral surface of a bell cup in addition to a feed tube which is fitted in rotational shaft of the rotary atomizing head for spurting a thinner toward inner peripheral surface of the bell cup as in the above-described prior art coating machine.